A magnetic actuator device for actuating a moving member in a duct is known, in particular from document FR-A-2 879 339, the actuator device comprising a magnetic field generator external to the duct and a permanent magnetic field generator internal to the duct, separated from the external magnetic field generator by a non-magnetic separator wall, and connected to the moving member. In known magnetic actuator devices, the permanent magnetic field generator internal to the duct is mounted to slide with small clearance relative to the facing separator wall. When the fluid for packaging contains particles of metal, as occurs with sugar-or chocolate-containing fluids manufactured from solid ingredients that have been subjected to grinding while being prepared, the particles are attracted by the magnetic field and they thus come to press against the separator wall in register with the internal magnetic field generator and on the face of the internal permanent magnetic field generator that faces towards said wall. During the reciprocating displacements of the permanent magnetic field generator internal to the duct, friction between the particles and the wall runs the risk of creating wear and also of blocking the moving member when the metal particles accumulate and are jammed between the permanent magnetic field generator internal to the duct and the facing wall.